Conventionally, memory disks which are used to record and reproduce various types of information (letters, sounds and images, etc.) are composed of doughnut type metal substrates, the surfaces of which are finished with ultra precision, the substrate being, for instance( an aluminum alloy substrate sheet (substrate) covered with a magnetic substance. In order to precisely perform its duty of allowing recording and reproducing by the magnetic head, the surface of a memory disk is required to have a high degree of flatness and smoothness. Since the surface condition of a memory disk is dependent on the surface condition of its metal substrate, the metal substrate is required to have extremely high dimensional preciseness (flatness, such as the minimum possible amount of undulation and warping) and a high quality surface condition (defect-free, mirror-like smoothness).
Conventionally, there exist a number of ways of making these metal substrates for memory disks, for instance, (a) a process wherein machining is carried out ultra-precisely by a natural diamond bit, (b) a process wherein both sides are simultaneously ground with grinding stones, and (c) a process wherein both sides are polished with abrasive grains and hot water by use of a lapping machine. However, since all of the above processes require machining or grinding, problems have been experienced in that they involve a great deal of time resulting n low productivity and also in that they require many production facilities.